Differences in the decay rate of the colchicine-binding (microtubule) protein from chick enbryonic and adult brains have been discovered. These differences reflect a change in stability of the protein which may also reflect the changing demand for the protein during nerve growth. Attempts will be made to identify chemical changes in the protein occurring during the development of the chick brain by isolating microtubule protein from brains of different ages and characterizing them for nucleotide binding, subunit composition, carbohydrate content and by peptide mapping. Isolation microtubule protein will be assayed for in vivo phosphorylation and, if found, the peptide maps of this protein will be made and the phosphorylated peptides isolated and sequenced. Peptide maps of microtubule proteins isolated from brains of different ages will be checked for the presence of the phosphorylated peptide. The developmental changes seen in brain microtubule protein may be due to a change in state of the protein from one actively being incorporated into tubules to one in a static situation. Therefore, the turnover rate of the protein during developmental periods of rapid tubule synthesis and little or no synthesis will be measured. In addition, microtubule protein purified from nerve cell cultures will be analyzed before and after induction of fiber outgrowth to see if any changes in protein structure have occurred. The functional significance of the microtubule in energy dependent processes will be assessed by measuring the ATPase activity of the intact tubules and isolated tubule protein in the presence of other brain proteins with actin-like or myosin-like activity. Finally, the influence of nerve growth factor on microtubule assembly and transport processes in cultured ganglia will be assessed.